(a) Field of the Invention
This invention relates to hot pressed silicon carbide and more particularly relates to a hot pressed thermal shock resistant silicon carbide ceramic body which has high density, good electrical conductivity and good strength. The invention further relates to the method for manufacturing such a silicon carbide ceramic body.
(B) History of the Prior Art
In the prior art, silicon carbide ceramic bodies are generally made by one of two processes. One of the processes is hot pressing wherein particulate silicon carbide is pressed into a mold at high temperatures and pressures to form a shaped body. The other process is pressureless sintering wherein silicon carbide is preformed at low temperatures into a shape having the general shape of the finished body. The preforming is usually accomplished by pressing silicon carbide particles together at low temperatures. Subsequent to preforming, the body is heated to an elevated temperature approximately atmospheric pressure to form a finished silicon carbide ceramic body. Hot pressing has certain advantages over pressureless sintering. In particular, hot pressing results in a silicon carbide body having generally higher densities and greater strength than the pressureless sintered silicon carbide body. Furthermore, pressureless sintering is a two-step process wherein the silicon carbide must be preformed prior to heating whereas hot pressing combines the forming and heating step in one operation.
The use of neither hot pressing nor pressureless sintering has resulted in a silicon carbide body which has a density and strength as high as is desired. In addition, the use of neither hot pressing nor pressureless sintering has resulted in a high density silicon carbide body having good thermal shock resistance.
In the prior art, it has been discovered that the density of hot pressed silicon carbide bodies could be increased by incorporating certain additives such as boron carbide and boron nitride. The densities obtained through the use of the additives however, have not been as high as the theoretical density of silicon carbide nor has the thermal shock resistance of silicon carbide bodies obtained when these additives were used, been acceptable for many applications. Boron and boron carbide have been disclosed as hot pressing additives for increasing the density of silicon carbide in U.S. Pat. No. 3,960,577 and boron nitride has been disclosed as a hot pressing additive for increasing the density of silicon carbide in U.S. Pat. No. 3,954,483.